A pixilated, solid-state, high-resolution Positron Emission Tomography (PET) detector with individual readout for each pixel and higher efficiency for 511 keV photons than currently available PET detectors will be developed. Recent improvements in Constellation's mercuric iodide detector technology have lead to the production of 10 mm-thick detectors and pixilated detectors, which also provide depth-of-interaction information. Intrinsic spatial resolution of 1 mm is now possible with these new mercuric iodide detectors while retaining high efficiency. Increased spatial resolution is particularly important in the application of PET imaging to smaller living systems (e.g. mice used in research labs) or to detect smaller lesions (e.g. small tumors). Mercuric iodide detectors also provide improved spectral resolution by allowing discrimination between the two types of full energy events; photoelectric events deposit all of their energy in a single location while Compton scattering followed by absorption deposits the energy in multiple locations. Phase 1 results will demonstrate the feasibility and performance of the new detector technology. In Phase 2, a prototype coincidence detector will be constructed for clinical evaluation.